Board Thread:Pokémon Help/@comment-32228622-20170613201735/@comment-30155146-20170614193807

It's most important for speed, since its a 1/0 situation - you either move first or last.

For othear stats it's more percentage based - a  chansey may reqire defensive investment to not get x5ed by physical attackers, but for a mega salemence, it's stats are mostly 100+, so full EV investment will resault in 1.5x at best.